In the wake of the 68-year-old serial paedophile’s death, McCarney apparently told prison authorities that Black had confided in him that he had killed many children.

In court, Black never admitted to any of the murders he was convicted of.

It is understood that detectives from England flew to Northern Ireland last week to question McCarney at length on his claims.

McCarney, 36, from Trillick, County Tyrone, is serving a minimum 25-year sentence for murdering Enniskillen toddler Millie Martin in 2009.

Black, from Falkirk, was a delivery driver who stalked the roads of the UK searching for victims.

His reign of terror was ended in 1990 when he was caught red-handed by police with a barely alive six-year-old girl hooded, bound, gagged and stuffed in a sleeping bag in the back of his van in the Scottish village of Stow. He had sexually assaulted her moments earlier.

Once in custody, the predator was linked to a series of unsolved crimes in the previous decade.

In 1994, Black was found guilty of three child murders in the 1980s - those of 11-year-old Susan Maxwell, from the Scottish Borders, five-year-old Caroline Hogg, from Edinburgh, and Sarah Harper, 10, from Morley, as well as a failed abduction bid in Nottingham in 1988.

In 2011, he was found guilty of the 1981 murder of nine-year-old Jennifer Cardy, from Ballinderry, Co Antrim.

Black was also suspected of involvement in other killings and unexplained disappearances and had long been the prime suspect in the case of missing 13-year-old Genette Tate, who was last seen in a rural lane in Aylesbeare, Devon, in 1978.